Polymeric dispersions are applied to nonwovens to hold together the fibers of a web, such that the web exhibits elevated tensile strength or is self-sustaining. Suitable conventional dispersions include self-crosslinking polymers of vinyl acetate and ethylene with N-methylolacrylamide (NMA) incorporated into the polymer as a crosslinker. The NMA crosslinker provides enhanced wet strength and resistance to organic solvents; however these compositions tend to evolve formaldehyde which is perceived as a health hazard. Accordingly, numerous attempts have been made to reduce or eliminate formaldehyde in self-crosslinking products.
U.S. Pat. No. 4,449,978 to Iacoviello discloses the usage of N-methylolacrylamide in conjunction with acrylamide (for example, in a 1:1 molar ratio) as the crosslinking chemicals. This approach does not provide formaldehyde free systems, but it does provide systems with significantly lower levels of free formaldehyde compared to systems using only N-methylol-acrylamide as the crosslinking chemical. It is seen in this patent that the same amount by weight of the mixture provides properties comparable to N-methylolacrylamide only, even though only half of the moles of the mixture contain the methylol self-crosslinking moiety. See Table 4, Col. 13-14.
U.S. Pat. Nos. 3,714,099 and 3,714,100 to Biale and Biale et al. respectively disclose using N-ethylolacrylamide as the crosslinking monomer in either vinyl acetate or vinyl acetate/ethylene polymers. U.S. Pat. No. 5,021,529 to Garrett also discloses using N-ethylolacrylamide as an ingredient, or using acrylamide and then adding acetaldehyde to the finished dispersion polymer thereby making N-ethylolacrylamide in-situ.
While N-ethylolacrylamides have been disclosed in the literature to be useful as crosslinkers, they are not used commercially because, in part, they do not provide wet strength properties comparable to NMA containing compositions. See U.S. Pat. No. 5,021,529 to Garrett noted above at Col. 9-Col. 10, Tables 1 and 2 as to wet strength of ethyl acrylate binders, where it is seen that the NMA based self-crosslinking resin systems exhibit much more wet tensile. Thus, N-ethylolacrylamide and higher alkylolacrylamides are generally perceived as unsuitable as a crosslinker for commercial use.
Without intending to be bound by any particular theory, it is noted that the equilibrium of the reaction of acrylamide and acetaldehyde lies less towards the product, N-ethylolacrylamide, than the corresponding reaction of formaldehyde with acrylamide lies towards its product, N-methylolacrylamide. Therefore, there is inherently a relatively high level of acetaldehyde present in N-ethylolacrylamide compositions. Acetaldehyde (unlike formaldehyde) is a chain transfer agent which results in the formation of low molecular weight polymers or oligomers rather than the higher molecular weight polymers needed for Engineered Fabric and textile applications. In these applications, the goal is to achieve infinite molecular weight networks to provide the finished fabric with wet strength and solvent resistance comparable to woven fabrics. Since acetaldehyde in the polymerization mixture acts as a chain transfer agent, transferring the radical from the chain to a monomer, the resulting product fails to achieve sufficient molecular weight to provide the desired strength properties. Adding more N-ethylolacrylamide will provide a higher level of crosslinking, but also increases the amount of acetaldehyde which has a deleterious effect on molecular weight.
One proposed method for removing aldehydes from N-alkylolacrylamide mixtures involves reducing the aldehyde to ethanol with sodium borohydride. See U.S. Pat. No. 5,415,926 to Leighton et al. While this may be a feasible method of rendering an aldehyde containing mixture useable, the method is expensive and time consuming, at best. It may also not be effective because the reaction product, N-alkylolacrylamide, may simply undergo retro-reaction to restore an equilibrium composition when used in a binder composition.